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In a chilling exchange of e-mails on the eve of the space shuttle Columbia's destruction, top NASA engineers expressed fears of precisely the nightmare scenario that came to pass.

NASA has released exchanges of e-mails between engineers that set out various scenarios the day before Columbia disintegrated on re-entry on February 1, killing all seven crew and spraying debris across several U.S. states.

But the middle managers at the U.S. space agency decided against relaying the fears to the agency's top echelons because they were seen as too alarmist, according to the impression given by the released exchanges.

"If there was hot plasma sneaking into the wheel well, we would see increases in our landing gear temperature," wrote flight engineer Jeffrey Kling of the Johnson Space Centre in Houston, on the morning of January 31. "Ultimately, our recommendation in that case is going to be to set up for a bailout - assuming the wheel does not burn off before we can get the crew out,"

William Anderson, an engineer with NASA sub-contractor United Space Alliance, responded that afternoon: "First, why are we talking about this on the day before the landing and not the day after launch?" The message exchange reflects serious fears by some specialists that the shuttle's ceramic tiled thermal heat shield could suffer heavy damage on re-entry.

One of the first signs of these fears came in a January 30 message from Robert Daugherty, an engineer at NASA's Langley Research Centre in Virginia, who wrote to an NASA official at Houston: "You should seriously consider the possibility of the (landing) gear not deploying at all if there is a substantial breach of the wheel well. At some point, the wheel could fail and send debris everywhere."

On January 27, as the Columbia was still orbiting Earth, Daugherty wrote: "Seems to me the benefit of an EVA (extra-vehicular activity, or spacewalk) to go out and look at the damage has more pros than cons. Can't imagine that an astronaut would cause MORE damage than he is going out to look for."

But after debate by e-mail and telephone, the engineers at Langley decided, on the eve of the doomed shuttle's return, not to convey their fears to NASA's top management.

On the following day, the tragedy they envisioned came to pass almost exactly as they had imagined, by an abnormal overheating and a breakdown of the temperature monitoring system in the Columbia's left wing, minutes before it disintegrated over Texas.

Debate among NASA engineers and researchers over possible damage to the shuttle's thermal shield began with the discovery, shortly after the launch of the 16-day mission, that a piece of foam insulation, possibly laced with ice, had broken free from the shuttle's external fuel tank and hit the thermal shield tiles below the left wing or along its leading edge.

The independent commission investigating the cause of the disaster did not immediately respond to the release of the message exchange.